1. Field of the Invention
The present invention relates to frequency-converted solid state lasers that include an intracavity non-linear solid state element for frequency conversion.
2. Description of Related Art
It is well known that intracavity-doubled diode pumped solid state (xe2x80x9cDPSSxe2x80x9d) lasers often exhibit chaotic amplitude fluctuations, spiking, and hysteresis. One explanation of this behavior relates to two nonlinear couplings-spatial hole burning and sum frequency generation-between different lasing modes. This chaotic noise problem appears in many diode-pumped solid state lasers at all wavelengths, although it is sometimes termed the xe2x80x9cgreen problemxe2x80x9d. Prior techniques proposed to avoid chaotic noise in intracavity-doubled lasers include:
1. Operating the laser in a single longitudinal mode. One disadvantage of this approach is that it generally requires frequency-selective elements in the laser cavity, increasing cavity loss, cost, complexity and size.
2. Operating the laser in hundreds of longitudinal modes, resulting in an averaging of noise. This approach generally requires a long cavity, and noise is reduced only as the inverse square root of the number of modes oscillating.
3. Eliminating spatial hole-burning by enforcing circular polarization within the gain medium, which requires a non-birefringent gain medium and additional intracavity polarization-control elements. This approach is not suitable for a gain medium such as Nd:YVO4, which is highly birefringent.
4. Two-mode operation: situating the gain and nonlinear crystals at specific points in a standing-wave cavity where it is believed spatial hole-burning and sum frequency generation between the two oscillating modes are reduced, such as disclosed in U.S. Pat. No. 5,627,849.
5. Phase-locking: arrange for oscillation in a set of phase-locked modes to avoid chaotic noise. The conditions necessary to reliably achieve phase locking are not clear, and therefore phase-locked lasers typically exhibit inconsistent performance.
6. Eliminating the nonlinear coupling due to sum-frequency generation by altering the polarization in the nonlinear crystal. This approach requires a separate quarter-wave plate, which adds cost and complexity to a system, and increases losses.
All of these approaches have disadvantages, especially when used with a birefringent gain crystal such as Nd:YVO4.
Laser gain media typically comprise a crystalline structure, in which the crystal axes are specified by three axes: the a-axis, b-axis, and c-axis. In order to obtain high efficiency in a birefringent gain medium such as Nd:YVO4, the intracavity elements of the laser are typically arranged in such a way that the fundamental emission is linearly polarized in alignment with the c-axis throughout the laser crystal. Therefore, in conventional solid state laser systems there is substantially no polarization change within the Nd:YVO4 crystal.
In order to align the intracavity laser beam along the c-axis within the laser crystal in one conventional implementation, the nonlinear crystal is temperature-tuned to provide a one-pass polarization retardance of one-half wave, which translates to a total cavity retardance of one full wave outside of the laser crystal, thus ensuring that the intracavity laser beam will be linearly polarized along the c-axis of the laser crystal. However, if two or more longitudinal modes are oscillating in the laser, this configuration is susceptible to so-called xe2x80x9cgreen problemxe2x80x9d chaotic noise since these two modes are forced to oscillate substantially along the same path; accordingly such lasers are typically designed for single mode operation. Unfortunately, single mode operation typically magnifies spatial hole burning problems within the gain medium, which destabilizes the fundamental emission.
The device described herein provides a way to reduce or eliminate noise in intracavity-doubled lasers in a very simple and compact package. Particularly, the device may reduce or eliminate noise that may be caused by chaotic amplitude fluctuations, spiking and hysteresis, such as may be caused by spatial hole-burning and sum frequency generation nonlinear coupling between different lasing modes. Embodiments are described in which noise reduction is provided by arranging the gain crystal and nonlinear crystal to provide a one pass polarization retardance about one-half wave or an odd multiple thereof, thereby eliminating the need for additional cavity elements, simplifying production requirements, reducing costs, and promoting higher efficiency in some embodiments.
A laser described herein includes a birefringent gain crystal arranged in a cavity to rotate polarization of the fundamental emission, resulting in a polarization that is not constant within the gain crystal. It is believed that the polarization rotation of the two modes within the gain crystal substantially reduces or even eliminates spatial hole burning.
A low-noise laser that provides a laser beam at a predefined lasing wavelength comprises a laser cavity and a birefringent gain crystal such as Nd:YVO4 situated within the laser cavity. The gain crystal is configured so that the fundamental laser emission within the laser crystal has a non-constant polarization, thereby providing a first predetermined nonzero amount of polarization retardance at the lasing wavelength. An optical element is situated within the laser cavity, the optical element configured to provide a second nonzero predetermined amount of polarization retardance that, in conjunction with the birefringent gain crystal, provides a total single-pass retardance for the laser cavity that limits laser noise within the laser cavity. In some embodiments the total single-pass retardance is approximately equal to an odd multiple of one-half wave; for example if the first predetermined retardance is approximately one-quarter wave, then the second predetermined retardance is approximately one-quarter wave. The number of longitudinal cavity modes is limited to substantially two, and each mode tends to follows a different polarization within the laser cavity.
In one embodiment the optical element comprises a nonlinear crystal situated within the laser cavity for Type II doubling. The principal crystal axis of the gain crystal is oriented in a first direction orthogonal to the axis of the laser cavity, the principal axis of the nonlinear crystal is oriented in a second direction orthogonal to the axis of the laser cavity, and the first and the second directions have a nonzero offset angle from each other. The offset angle may be within a range of about 30xc2x0 to about 60xc2x0 for example about 45xc2x0.
In an alternative embodiment, the optical element comprises a quarter-wave plate. In one such embodiment the laser output comprises the lasing wavelength, including substantially a first linear polarization and a second linear polarization that is orthogonal to the first linear polarization.
In one embodiment the gain crystal and nonlinear material are coupled together, such as by bonding or optical contact, to form a monolithic component. Low-noise operation can be achieved with the monolithic component over greater than 10xc2x0 C. temperature ranges. This low-noise operation is also largely insensitive to pumping conditions (pump laser power, wavelength, spot size) and is expected to be scalable to high output powers. The low-noise laser described herein is therefore well-suited to applications that require a low-cost, rugged, and compact source of laser light.